1. Field of the Invention
The present invention relates to a sheet transport apparatus and an image forming apparatus.
2. Description of the Related Art
Conventionally, image forming apparatuses such as copiers, printers and facsimile machines include an image forming portion and a sheet transport apparatus that transports sheets to the image forming portion by transport rollers. In the conventional image forming apparatuses, when a sheet is transported, the sheet may skew because of, e.g., deformation or misalignment of the transport rollers. In the image forming apparatuses, the accuracy of an image forming position relative to a sheet greatly depends on the position of the sheet relative to the image forming portion. Therefore, accurate positioning of a sheet relative to the image forming portion is an important factor for image quality.
In the conventional image forming apparatuses, a skew correction portion is provided in the sheet transport apparatus. Skew of a sheet is corrected by the skew correction portion to enhance the image forming position accuracy. Examples of such image forming apparatuses include an apparatus that includes a shutter member biased in a direction opposite to a sheet transport direction by, e.g., a spring as a skew correction portion (see Japanese Patent No. 3768576).
With such shutter-type skew correction apparatus, a transported sheet abuts against an abutting portion of the shutter member to form a loop. Consequently, a leading edge of the sheet follows the abutting portion, whereby skew of the sheet is corrected. Ordinarily, the abutting portion of the shutter member against which a leading edge of a sheet abuts is made of a resin member.
In conventional sheet transport apparatuses including a shutter member, when a leading edge of a sheet abuts against an abutting portion of the shutter member, the leading edge of the sheet then moves in a width direction perpendicular to a sheet transport direction so as to follow the abutting portion while pressure contacting with the abutting portion. At this time, if the abutting portion of the shutter member is made of a resin member, when the leading edge of the sheet moves in the width direction while contacting with the abutting portion, a part of the abutting portion against that the leading edge of the sheet abuts is whittled by the sheet and a groove portion is formed.
If the groove portion is formed in the abutting portion of the shutter member, when other sheets are subsequently transported, the groove portion is gradually deepened. As a result, a leading edge of a sheet may be caught in the groove portion, causing a transport failure. As described above, when an abutting portion of a shutter member is made of a resin member, a groove portion may be formed in the abutting portion of the shutter member during a long period of use, and if a leading edge of a sheet is caught in the groove portion, a transport failure may occur.